As an advertising medium, a digital signage system utilizes digital technology in the images to be displayed and in the control of the display. For example, digital signage systems are installed on the walls of department stores, supermarkets, and other commercial facilities, and in railway signs, road signs, and signs at other public facilities. Image content may be displayed by compressing digitized video data by a coding technique complying with a standard such as MPEG-2 or H.264, distributing the compressed video data through a network etc., and temporarily storing the compressed video data before reproduction. Alternatively, reproduction follows distribution immediately on a real-time basis. Digital signage systems can provide a wide variety of image expression: a large screen may be configured by use of multiple display units in order to enable distant viewing, or a plurality of display units may be arranged horizontally or vertically, in such a manner as to suite the installation site, and their individual displays may be linked cooperatively.
One proposed method of image display synchronization to be used when an image is displayed on a plurality of display units in the way explained above, and when a single image is divided up for display on the individual display units, is to store the image data in a memory as digital video data and have image processing units corresponding to the individual display units read the digital video data in synchronization (see Patent Reference 1, for example).
In another method having been proposed for synchronizing digital video signals to be decoded in a plurality of individual playback units for reproducing digitally encoded content data, when a screen is displayed by connecting the playback units to corresponding display units, one of the plurality of playback units is used as a master playback unit and the rest are used as slave playback units, and the master playback unit transmits a reference clock to the individual slave playback units, which decode the data in synchronization with the clock transmitted from the master playback unit in order to display the screen (see Patent Reference 2, for example).